This invention relates generally to the field of mixing or compounding reinforcing fibers with thermoplastic resin.
The fibers may be of any predetermined length and are introduced into admixture with thermoplastic resins for immediate use in molding machines at a product molding facility.
Processes are known for combining reinforcing fibers with thermoplastic resin to form reinforced plastic composites. U.S. Pat. Nos. 4,439,387 and Re. 32,772 sharing common inventorship herewith disclose the embedding of thermoplastic resin in reinforcing fibers as they are drawn through a forming die having a convoluted passage, in the presence of molten thermoplastic resin introduced from an extruder. The extrusion product of that process comprises an elongated bar or rod having a continuous length of reinforcing fiber encased within thermoplastic resin. That preformed composite may be inserted into a die of an injection molding machine, and utilized as an insert in a compound, composite product comprising an additional layer of thermoplastic resin molded over the insert. It is also known to cut the extrudate rod from the aforesaid forming die into short lengths for use as molding pellets. In the process of the aforesaid United States patents, the extruded rod comprising a fiber/resin composite is immediately cooled, prior to final forming and cutting to desired lengths.
U.S. Pat. No. 5,185,117, also having identity of inventorship herewith, discloses a process for compounding thermoplastic resin and fibers in an extruding compounder. According to the process of that patent, melted thermoplastic resin is introduced into the compounding extruder along with reinforcing fibers. The resulting extrudate consists of a molten mass of thermoplastic resin having discrete lengths of fibers randomly dispersed therein. This hot mixture may then be fed directly into a preform device to produce a measured preform for use in a compression molding machine. In the disclosed process of the ""117 patent, the fibers are precut to desired lengths, before being introduced into the extruding compounder. The process and apparatus further requires a loss-in-weight scale to accurately measure predetermined quantities by weight of reinforcing fibers to be controllably introduced into the compounding extruder in the presence of thermoplastic resin. A separate loss-in-weight feed scale assembly is required to accurately convey predetermined amounts by weight of the thermoplastic resin into the compounding extruder for mixing in the desired proportions with the reinforcing fibers. The loss-in-weight scales necessarily add to the cost and complexity of the compounding apparatus.
U.S. Pat. No. 4,616,989 discloses an apparatus for incorporating glass fibers into thermoplastic resins in which a premixing chamber is utilized to initially mix glass fibers with molten resin. This mixture is then fed into a two-stage screw-type extruder to complete the mixing of the fibers and resin. The resin-fiber mass as discharged from the final extruder is passed through a forming die having a plurality of orifices. This serves to form the material into elongated filaments of glass fiber reinforced resin which are then cooled, and then granulated for use as a molding compound.
U.S. Pat. No. 2,877,501 to Brandt discloses a process for forming granules comprised of glass fibers coated with a molding plastic, which are intended for use as feed stock for an injection molding machine. In the Brandt process, fibers are pulled through an orifice within which they are coated with a resin material.
None of the known prior art processes for formulating a mixture of fiber reinforced resin for molding purposes are operatively effective for preparing a molding material comprised of fiber reinforced resin for immediate introduction into a molding machine at the same site where the fiber-resin mixture is made. Either cumbersome and costly apparatus, including multiple stage mixing devices and loss-in-weight scales, are required in the prior processes, and/or the fiber-reinforced resin is extruded into lengths, cut and packaged for sale and transportation to separate molding facilities.
There thus exists a need for a compact, efficient apparatus and process for accurately formulating mixtures of fiber and resin and thereafter directly introducing that molding material directly into a molding machine, such as an injection molding machine, a compression molding press, a transfer mold, a blow mold, a profile extrusion machine or an inject compression molding machine.
This invention has as its primary objective the provision of a process and apparatus for interspersing fibers in a thermoplastic resin in a desired weight ratio, under a controlled, fiber-coating process, and thereafter introducing the coated fibers directly into a molding machine at the same site, without cooling of the fiber-resin mixture.
This basic objective is realized by conveying at least one fiber strand through a die chamber in the presence of molten thermoplastic resin, thereby coating the fiber strand with the resin; and thereafter directly introducing the resin-coated fiber strand in a heated state into a receiver for movement into a molding machine. The receiver may comprise an extruder barrel which houses a rotatable screw or simply a plate or tray utilized to convey a pliant mass of resin and fiber into the mold press of a compression molding machine.
The fiber strand may be cut into predetermined lengths to form a hot molten mixture of resin and fibers for use in a molding machine.
The molding machine may comprise any type of molding apparatus, such as an injection molder or a compression molder. In the former case, the aforesaid extruder comprises the barrel containing a rotatable screw serving as the in-feed device for forcing molding material into the mold cavity of an injection molding machine. In this embodiment, the coated fiber strands are conveyed directly into the barrel of the in-feed extruder after being coated.
Alternatively, the extruder into which the coated fiber strand is introduced may comprise a compounding extruder as disclosed in the aforesaid U.S. Pat. No. 5,185,117. The final impregnating and wetting of the fibers by the resin takes place in that compounding extruder. As disclosed in U.S. Pat. No. 5,185,117, the molten mass of thermoplastic resin and fibers discharged from the compounding extruder may be preformed into a pliable mass which is conveyed to the mold press of a compression molding machine at the same site.
In both of the aforesaid embodiments, the final wetting and impregnating of the fibers with thermoplastic resin takes place in the screw barrel of the extruder into which the coated fiber strands are introduced.
As a particularly advantageous feature, a mechanical conveying device is utilized to pull either a single fiber strand or multiple strands of fibers through a coating die as described above, into which molten thermoplastic resin is introduced from a separate extruder. The same conveying device serves to push the coated fibers into a receiving device as stated above, such as the barrel housing an extruder screw for subsequent injection or compression molding.
In the preferred embodiment, a cutting device is provided at the output site of the conveying device. The coated fiber strand(s) is cut into a plurality of predetermined lengths at the discharge side of the conveying device to thereby form a hot molten mixture of resin and fibers for use in a molding machine. The conveying device generates pressure which serves to push that hot molten mixture of resin and fibers into the barrel of a screw-type extruder as described above.
The conveying device may comprise a mechanical pump in the form of a pair of cooperating screws having impeller blades which grip the fiber strand and pull it through the coating chamber and thereafter push it into the extruder barrel. Alternatively, the pump may comprise a gear pump having a pair of cooperating gears with intermeshing teeth; or, the pump may comprise cooperating rollers defining a nip therebetween within which the fiber strand is gripped and pulled through the coating chamber and thereafter pushed through the output side or port of the pump into the extruder barrel.
The aforesaid coating chamber may advantageously comprise one or more orifices of a predetermined size larger in cross-sectional area than the cross-sectional area of the fiber strands to be coated. Multiple fiber strands may be passed through such orifices, with the annular space between the strand and each orifice permitting the predetermined amount of molten resin to pass through with the fiber, by means of which the amount of resin coating each fiber strand may be controlled. The fiber strands will normally comprise bundles made up of a plurality of fine fiber filaments.
In any of the aforesaid embodiments, the fiber is caused to be heated by admixture with the molten resin in the coating chamber, with the fiber being further heated by the mechanical pumping action of the conveying device. The coated fiber is forced into the extruder barrel or onto a transport receiver in a heated state substantially immediately after being discharged from the conveying device.
The aforesaid apparatus and process may be controlled in coordination with the operation of an extruder into which the coated fibers are introduced for direct molding. In applications where the extruder screw comprises the in-feed screw of an injection molding machine, that screw is intermittently actuated to intermittently feed charges of molding material into the injection mold. The aforesaid conveying device, in whichever form of mechanical pump is utilized, is also intermittently actuated in synchronization with the intermittent operation of the rotatable screw of the in-feed extruder to the injection molding apparatus. This ensures that the supply of fibers and resin in a molten mass is accurately fed into the barrel of the extruding screw, only when that extruding screw is rotating to receive such material and convey it forwardly toward its discharge end. No fiber and resin mix is fed into the extruder barrel when it is operating during the in-feed phase of its cycle in which it is conveying a charge of fiber and resin molding material into the molding chamber of the injection molding machine.
As a further control feature, the screw of the extruder utilized to supply a molten thermoplastic resin to the coating chamber may also be intermittently actuated in coordination with the intermittent rotational operation of the screw housed within the barrel into which the resin-coated fiber is directed.
These and other objects and advantages of the invention will become readily apparent as the following description is read in conjunction with the accompanying drawings wherein like reference numerals have been utilized to identify like elements throughout the several views.